1. Field of the Invention
This invention relates to a system and method for shaping an antenna radiation pattern, and more particularly to a system and method of sidelobe reduction in the principal planes.
2. Description of Related Art
An antenna is a structure for transmitting or receiving radiowaves. It is typically desirable for an antenna to have isolation from antennas other than antennas in a particular direction. In general the particular direction will be towards a target antenna. For many applications, effective isolation means that it is desirable for the antenna to have a particular beam shape, e.g., a shape with selected side lobe suppression.
If the antenna is transmitting, appropriate side lobe suppression makes it less likely that the antenna will interfere with the operation of other antennas. If the antenna is receiving, such suppression makes it less likely that the antenna will be susceptible to interference from other antennas. In an environment where interference from other antennas may be intentional, such as an environment where there is jamming of radar or communication systems, side lobe suppression may be an important part of an electronic counter measures capability.
It may desirable to suppress side lobes in a certain plane of the radiation pattern of an antenna. In an environment where both the target antenna and the other antennas are on a common horizon, for example, sidelobes should be suppressed in a principal plane aligned with the horizon
One principal plane of an antenna is that plane containing both the electric field vector and the direction of maximum radiation. Another principal plane is that plane containing both the magnetic field vector and the direction of maximum radiation.
Another environment where it is desirable to suppress sidelobes in a certain plane is in a geosynchronous orbit satellite communication system. Assuming ground antennas are located at the equator, sidelobes should be suppressed in a principal plane aligned with the equator. If the ground antennas are not located at the equator, side lobe suppression in a principal plane still provides isolation from the other antennas when the other antennas are located relatively close together.
An example of a planar array system is shown in FIG. 1(a). This example consists of a radio signal processor 120, a feed network 130, and an antenna array 170 consisting of 199 columns of elements in azimuth and 39 rows of elements in elevation. A front view of array 170 is shown in FIG. 1(a) and a top view in FIG. 1(b). The system of FIGS. 1(a)-(b) has a -50 dB Taylor Distribution, Nbar=10, and an element spacing of 0.5 wavelengths with Cos(Theta) element factor. FIG. 2(a) is a plot of a radiation pattern of the depicted antenna. FIG. 2(a) shows a radiation pattern in a principal plane having a varying azimuth and running parallel to each set of 199 elements. The horizontal axis in FIG. 2(a) is the angle relative to an axis perpendicular to the center of the array, with 0 degrees looking straight out of the center of the array. The highest peak centered about the azimuth of zero degrees is called the main lobe. The peaks on either side of the mainlobe are sidelobes. It is desirable for sidelobes to be small or well-controlled relative to the main lobe.
FIG. 2(b) is a plot of another principal plane pattern, perpendicular to that of FIG. 2(a). FIG. 2(b) shows a radiation pattern of the planar array in a principal plane having a varying elevation, with 0 degrees looking straight out of the center of the array.
FIG. 2(c) is a three-dimensional grid contour from -20 to +20 degrees in both azimuth and elevation. As can be seen from FIG. 2(c), the main areas of sidelobe energy are located in two bands centered on the principal planes of the antenna. The antenna radiation patterns depicted in FIGS. 2(a)-(c) are the result of constructive and destructive interference between the elements of the antenna.
There have been a few methods developed for reducing sidelobe energy. One method involves amplitude tapering, in which the amplitude of the elements in the center of the array is high relative to the elements near the periphery. Amplitudes are set at the design level for sidelobes next to the main beam and taper off with an approximate sin x/x dependency from there out to the periphery of the array. The radiation patterns shown in FIGS. 2(a)-(c) are the result of amplitude tapering. As described earlier, this means that the highest sidelobes are in the principal planes and adjacent to the main beam.
Other methods of reducing sidelobes are typically achieved through the use of a sidelobe canceller or adaptive nulling.